Resistance apparatus



ly 1957 c. R. DICKINSON ET AL 2,798,139

RESISTANCE APPARATUS Filed Oct. 14, 1955 2 Sheets$heet l \NVENTORS MZQM WZW ATTORNEY July 2, 1957 c. R. DICKINSON ET AL 2,798,139

RESISTANCE APPARATUS Filed Oct. 14, 1955 2 Sheets-Sheet 2 AT TORNEY United States Patent QfiFice 2,798,139 Patented July 2, 1957 RESISTANCE APPARATUS Clarence R. Dickinson, Greendale, and Bernhard F. Tellkamp, Muskego, Wis., assignors to Allen-Bradley Company, Milwaukee, Wis., a corporation of Wisconsin Application October 14, 1953, Serial No. 385,952

8 Claims. (Cl. 201-48) This invention relates to variable resistance apparatus such as is commonly employed for rheostats and potentiometers and resides more specifically in variable resistance apparatus that may be compacted to within small overall dimensions for use in circuits having a great number of components confined within small space limits, or in other instances where compactness or light weight are of dominating concern.

The invention relates primarily to so-called integrally molded variable resistance apparatus in which there is provided a molded base of insulating material having a layer of dispersed conducting particles that form a resistance track integrally embedded therein and a similar layer of more densely distributed conducting particles that form a collector track of negligible resistance also embedded therein, the tracks each being insulated from one. another and exposed to one side of the base with concentrically arranged'contact surfaces. The contact surfaces are electrically bridged by a rotatably mounted brush of conducting material having a set of contact faces held in bearing engagement with the exposed track surfaces. Ends of terminal leads are embedded in the base in electrical contact with the tracks and extend therefrom for suitable connection with other circuit components or conductors. Molded apparatus, as here de scribed, has highly desirable operating and life characteristics and this invention increases the range of application of such apparatus.

It is an object of this invention to provide a variable resistance apparatus as described with high leakage resistance between the conducting portions of the apparatus and grounded metallic portions to permit a compacting to within small overall dimensions. Upon compacting, leakage resistance paths become foreshortened and maximum voltage ratings will be decreased accordingly. In the practice of this invention, however, there may be employed a novel arrangement of insulating material that jackets the conducting circuit elements in manner to retain leakage resistance at high values heretofore had with larger apparatus.

It is another object of this invention to provide a variable resistance apparatus with terminal leads mounted to withstand substantial abusive handling of the outer ends to which connections are made without dislocation of the embedded ends in electrical contact with the resistance and collector tracks.

It is another object of this invention to provide a variable resistance apparatus with a sliding brush in contacting relation with resistance and collector tracks that has a substantially constant contact-pressure between brush and tracks throughout the full range of brush movement and over an extended period of use.

It is still another object of this invention to provide a variable resistance apparatus as described that may be compacted within small overall dimensions without un-' desirable decrease in power rating heretofore had in larger apparatus. To retain wattage ratings comparable with that of larger apparatus, while permitting reduction in overall dimensions, a heat conducting element is embedded within the molded insulating base of which the resistance track is a part. This element extends to within close proximity of the resistance track to provide a conductive path for the heat generated within the track and extends to the exterior of the base to a heat conductive connection with portions of a metallic covering to readily dissipate the heat conducted away from the resistance track.

These and other objects and advantages of this invention will appear in the description to follow. In the description reference is made to the accompanying drawings in which there is shown by way of illustration and not of limitation a specific form in which the invention may be embodied.

In the drawings:

Fig. l is a side view in elevation and in section of a variable resistance apparatus in which this invention is embodied,

Fig. 2 is a rear view in elevation and in section of the apparatus viewed through the plane 2-2 indicated in Fig. 1,

Fig. 3 is a front view in elevation and in section of the apparatus viewed through the plane 3--3 indicated in Fig. 1,

Fig. 4 is a top plan view of the apparatus shown in Fig. 1,

Fig. 5 is a view in perspective of a subassembly that forms a portion of the apparatus,

Fig. 6 is a view in perspective of a brush carrier and operating shaft forming a part of the apparatus,

Fig. 7 is a front view in elevation of the brush carrier and operating shaft assembly shown in Fig. 6,

Fig. 8 is a fragmentary side view in elevation with parts broken away and in section of the brush carrier and operating shaft assembly viewed through the plane 8-8 indicated in Fig. 7,

Fig. 9 is a view in perspective of a heat conducting insert that forms 'a part of the apparatus and which is embedded within the molded base, and

Fig. 10 is a view in perspective of an electrically conducting brush that forms a part of the apparatus.

In the embodiment of this invention shown in the drawings, which is a variable resistance apparatus suitable for use as a potentiometer or rheostat, there is provided a base 1 of molded insulating material having a circular track 2 of conducting resistance material and a circular collector track 3 likewise of conducting material, each track being integrally molded with the base 1 to form a unitary whole. The base 1 is preferably formed of a molded synthetic resin having desirable insulating qualities that electrically separates the track 2 from the track 3. The resistance track 2 comprises a shallow layer exposed on the rear face of the base 1 of finely divided conducting particles of carbon black distributed to obtain desired resistance characteristics. Similarly, the conducting track 3 is composed of a shallow layer exposed at the rear of the base 1 of conducting carbon black particles that are more densely distributed than in the track 2 to provide a conducting path having a negligible resistance. Apparatus employing conducting particles molded within an insulating base are termed and disclosed in detail in Patents No. 2,269,136 issued to Bernhard F. Tellkamp on January 6, 1942 and No. 2,514,682 issued to Bernhard F. Tellkamp on July 11, 1950.

Embedded within the base 1, .at the time of molding, is a set of three terminal leads comprising a center lead 4 with an embedded end extending to within the collector track 3, as shown in Fig. 1, to form an electricalconnection, and a'pair of outer leads 5 disposed to each side of the center lead 4 with embedded ends in electricaily conducting relation with the ends of the resistance track 2. The embedded ends of the leads 4, 5 are in initimate contact with the resin forming the base 1 and are of a configuration to be'securely anchoredto resist being pulled or twisted out of position, unless an excessive abusive force be applied. From the embedded ends each lead 4,5 emerges into a central shank portion 6 and hence into a free end 7 suitable for connection with conductors extending to other circuit elements. Connection may readily be made by soldering and the leads .4, '5 .are formed of a conductive material such as plated copper wire that .is highly bendable, to enhance the ease with which such connections may be made.

Also embedded within the'base 1 at the time of molding is a heat extracting element '8 of highheat con-1 ductivity formed as shown .in 'Fig. 9. The element '8 is curved to present 'an after edge 9 that describes a partial circle which is disposed in close proximity to the resistance track 2, as shown in Fig. 1. The element 8 provides a path to conduct heat generated within the resistance track 2 away from the interior of the base 1, but is spaced sufiiciently far from the track 2 to maintain a desired high value leakage resistance to retain a high breakdown voltage rating.

The front .margin of the element 8 is provided with a plurality of cars 10, as shown 'in 'Fig. 9, which are inserted through mating openings 11 in a front plate -12, after molding is completed, and then bent over to clinch the base 1 and plate 12 together. The plate 12 has a dished central area in which the cars 11) are received and the contacting engagement between the cars and plate 12 forms a heat conducting path tolead heat away from the element 8.

Clampingly held between the front face of the base 1 and the front .plate 12 is amolded insulating shell 13 with a central opening in which the dished portion of the cover plate 12 is seated. 'The shell 13 turns about the cylindrical side of the base 1 to extendrearwardly in the form of 'a rim 14, as 'is clearly shown-in Figs. 1, .2.and 5. The rim 14 is spaced from the base 1 and extends .to

the rear of the face of the base 1 upon which the tracks- 2 and 3 are exposed. As is more clearly shown in Fig. 5, the rim 14 does not fully encircle the base 1 and at the upper portion of the shell 13, where the rim 14 is discontinued, there rises a supporting insulating wall 15 adjacent'the shank portions 6 of the leads 4, .5. The wall 15 is an integral part of the shell 13 and forms an insulating barrier between the leads '4, 5 and the front plate 12.

Riveted to the front plate 12.is .a bushing 16 threaded to receive suitable mounting hardware, not shown. Extending through the bushing 16 is an operating shaft 17 with an inner end 18 of reduced diameter and disposed on the inner end 18 is a molded ibrush carrier 19. The carrier 19 has a circular front face 20 of a diameter substantially the same as that of the .rearface of the base 1, so that upon inserting the shaft 17 through the bushing 16 the brush carrier 19maybebroughtupin facing relation to the base 1 to cover the tracks 2 and 3. In this position the carrier '19 fits within the .rim 14 of the insulating shield 13 that extends rearwardly-of the base 1, as shown in Fig. .1. A collar 41, forming a part of the carrier 19, encircles the shaft 17 andextends to the front of the face 20 to be received within the central 4 opening of the base 1 and to electrically shield the collector track 3 from the shaft 17.

Held by riveted engagement to the after end of the shaft 17 is a disc 21 with a forwardly turned car 22 extending within a recess 23 in the brush carrier 19. R0- tation of the shaft 17 will then cause the brush carrier 19 to be rotated in like direction. The disc 21 is formed with a radially extending lug portion 24 that lays upon a portion of the carrier 19 having a like configuration. The lug 24 cooperates with a stop, to be described that limits the rotation of the shaft 17.

As shown in Figs. 7 and 8 the brush carrier 19 is recessed to house a conducting brush 25 and a staple socket 28 in the carrier 19. The two leg portions 43 extend over abutments 29 that project from the floor of the recess within the carrier 19 and .upon insertion of the brush 25 within the recess the spring 26 is bent about the abutments 29 into the dotted position of Fig. 8. The abutments 29 .provide a fulcrum about which the spring 26 may be turned and the leg ends 27 bear against the socket walls for a reaction to the bending distortion of the spring '26 caused by the insertion of the brush 25. I a

As shown inFig. 10, the brush 25 has a single foot.30 at one end for engagement with the resistance track 2, and a pair of collector feet .31 at the opposite end for engagement with the collector track 3. A conductive carbon material is employed for the brush 25 .to provide a low resistance conductor for bridging the tracks 2 and 3. The carbon material composing the brush must not rub off upon the tracks 2 and 3 to form a superimposed conducting layer .and .it must not scratch or otherwise mar the track surfaces. Such a brush has been described in greater detail in the patent to Tellkamp, .No. 2,514,682. With the'brush 25 and spring26 inserted within the carrier 19 and the shaft 17 passed through the bushing 16 to bring the carrier 19 into close facing relation with the tracks 2 and '3 a retaining-ring 32, that bears against a washer 33 at the front .of the bushing 16, is placed in a mating groove in the shaft 17 to retain the elements in assembly. 'With the elementsin assembly the brush 25 is retained within the carrier recess to distort .the spring 26, which in .turn acts upon .the brush to provide desired "bearing pressure between the brush feet 30, :31 andthe tracks "2, 3..

A cylindrical metallic cover 34,.closedat the rear and openat, the fronthouses the ,base .1 ,and carrier 19. .Aset of cars 35 extending-from the jfrontedge of .the cover -34 are turned over the frontplate 12 to .secure the cover 34 in .position. A portion of the .cover 34 at the top and front is notched to-receive a molded terminal guide 36 of insulating material that extends from within the cover 34to 'the exterior. The-terminal guide 36 has a circular groove 44 formedin the rear-face to seat upon a lip 37 defining -the;notch:in the cover 34. The groove 44 forms a ridge 38 that extends along the inner side of the cover 34 as an extension oflthe rim -14-of-theinsulating shield 13. As shown in Figs. 1 and 2, the rim 14 and the ridge 38 together form a circular insulating shield fully surrounding the gap-between the base '1 and thebrushholder 19. This shield, spaced bothfrom the metallic cover'34 and the base 1 providesia'high resistance leakage path between the conducting resistance track 2' and the cover 34, which will normally be.at near ground potential. .As clearly shown inFigs. 1-4, theterminal guide 36 is adjacent to and fits in mating relation with the supporting wall 15.of.the.shel1.13. .The guide 36 and wall 15 snugly surrounda substantial length of-theshank portions :6 of the terminalleads-4,.5 that emerge'from the base 2,,an d aredisposed to support .the ieadsA, 5 .as reaction .mem-

bers upon side thrusts being applied to the free ends of the leads 4, 5. I

Riveted to the cover 34 in a position to obstruct the lug portion 24 of the disc 21 is a stop 39. The stop 39 limits the rotation of the shaft 17 and the brush 25 to prohibit the contacting foot 30 on the brush 25 from riding beyond the limits of the resistance track 2, which is in the form of a partially complete circle. For mounting the apparatus on a panel board, or the like, a pair of fingers 40 are provided on the cover 34 that extend forwardly, as shown, to be adaptable for insertion in openings provided in the panel upon which the apparatus is to be mounted. The apparatus will then be orientated in position and will be restrained from twisting upon rotation of the operating shaft 17.

The form of the apparatus of this invention is well adapted for compacting into a small unit to provide a variable resistance unit particularly useful in aircraft and other applications where space and weight are premium factors. Upon compacting, the dissipation of the heat arising in the resistance track must be rapid if it is desired to retain wattage ratings formerly had in larger devices. To enhance the rate of dissipation in the apparatus of this invention the heat extractor 8 connects with the external metal portions of the apparatus comprising the front plate 12 and cover 34. Heat generated within the molded base 1 is then quickly conducted to the cover and the mounting panel which present large radiating surface areas for rapid heat dissipation.

To facilitate the compacting, bendable leads of small cross section are employed that may be turned and bent to suit the party making the electrical connections with conductors and other circuit components. To support the leads in fashion to resist excessive lateral stress and isolate the embedded roots of the leads in the base 1 from such stress the shanks 6 of the leads are snugly surrounded by supporting insulation. The insulation, in the form of the Wall 15 and the terminal guide 36, extends along a substantial length of each lead shank 6 to provide extended reaction areas against which the leads may bear in reaction to lateral stresses being applied thereto. The embedded roots will not be subjected to these lateral stresses and the possibility of fault appearing in the base 1 or between the base 1 and a lead 4, is precluded.

As has been noted herein, compacting of a variable resistance through the employment of this invention may be further enhanced by reason of the jacketing of the conducting elements within an insulation shield. The leads 4, 5 are surrounded by insulating members to retain high leakage resistance paths with the normally grounded protective front plate 12 and the cover 34. The resistance and collector tracks 2, 3 are insulated to the front by the base 1 and are jacketed to the rear by the brush holder 19 and to the peripheral sides by the rim 14 and ridge 38. To provide a high leakage resistance path between the operating shaft 17 and the inner edge of the collector track 3 the carrier 19 is extended within the central opening of the base 1 in the form of the collar 41. This pro tective jacketing, which in turn is encapsulated in a sturdy metallic cover, permits both compacting and usage where severe handling may be encountereo.

For reliable operation, resistance values for particular brush positions should be maintained consistent over an extended period of use. Consistency depends largely upon brush pressure determined by the spring 26. To maintain substantially constant spring pressure a spring with a flat spring rate characteristic within the range of deflection encountered should be employed. In this invention the spring is formed to present an extended active length comprising a central section that engages the brush and legs extending at angles thereto. This form permits compacting of the apparatus without forfeiture of beneficial spring length. The spring is held in fixed position by engagement of the toes 27 within the sockets 28 and by the spring legs being pressed against the abutmetnts 29. There is thus provided a variable resistance having characteristics including long life and uniformity of resistance values over a period of use wherein high leakage resistance and power ratings need not be sacrificedwith compacting of the apparatus.

We claim: I

1. In a variable resistance apparatus the combination comprising a molded base of insulating material having an exposed circular resistance track at the rear formed of conducting particles molded integrally within the material comprising said base and having side facing extending forwardly to the front, a brush carrier to the rear of said resistance track rotatably mounted for movement about an axis substantially concentric with said resistance track, a brush held in sliding engagement with said resistance track by said carrier and rotatable therewith, a bendable terminal lead having one end embedded at the time of molding within said molded base in electrical contact with said resistance track and having a shank portion emerging and extending outwardly from the side facing of said base terminating in a free end suitable for electrical connection with circuit components and leads, an insulating shell having a front wall and a rim extending rearwardly from the wall with a gap in the rim and a radial extension of the front wall at the gap, which shell receives the base with the front wall thereof immediately to the front of the base and the rim extending along the side facing of the base with the gap therein receiving the terminal lead which is disposed adjacent the radial extension of the front wall, a rim continuation of insulating material closing the gap in said rim and having a portion alongside said terminal lead that supplements said radial extension of the front wall whereby said radial extension and rim continuation form terminal lead supporting means of insulating material closely surrounding the shank of said lead for a substantial distance beyond the point of emergence of said lead to isolate the embedded end from bending forces applied to the free end of the lead, and mounting means retaining said molded base said insulating shell and said rim continuation in fixed relation.

' 2. In a variable resistance apparatus the combination comprising a molded base of insulating material having cylindrical side facing and a rear face exposing a circular collector track and a circular resistance track spaced from and substantially concentric with said collector track each formed of conducting particles molded integrally within said base, a brush carrier to the rear of said resistance and collector tracks rotatably mounted for movement about an axis substantially concentric with said tracks, a brush held in bridging contact with said tracks by said brush carrier and rotatable therewith, a set of bendable terminal leads having an end of each embedded at the time of molding within said molded base in electrical contact with one of said concentric tracks and having shank portions emerging and extending outwardly from the side facing of said base that terminate in free ends suitable for electrical connection with circuit component and leads, an insulating shell having a front wall and a rim extending rearwardly from the front wall with a gap in the rim and a radial extension of the front wall at the gap, which shell receives the base with the front wall thereof immediately to the front of the base and the rim extending about the side facing of the base with the gap therein receiving the terminal leads which are disposed adjacent the radial extension of the front wall, a rim continuation of insulating material closing the gap in said rim and having a portion alongside said terminal leads that supplements said radial extension of the front wall whereby said radial extension and rim continuation form terminal lead supporting means of insulating material closely surrounding the shank of each lead for a substantial distance beyond the point of emergence of each to isolate the embedded ends of the leads from bending forces, applied to the free ends thereof, and

arsed-a mounting means retaining said molded base, said insulating shell and said rim continuation in fixed relation.

3. In a variable resistance apparatus the combina i n comprising a -molded base of insulating material having a circular collector track ;and a circular resistance track spaced from and substantially concentric with said collector track ieach formed of conducting particles molded integrally within said base, a brush carrier rotatably mounted for movement about an axis substantially concentric with said tracks having a ,pairof abutments adapted to be spring fulcrums, ;a brush held by and rotatable with said brush carrier in bridging contact with said tracks, a spring interposed between said brush and said carrier having a straight center section bearing against the side of said brush opposite said tracks and a -pair;of leg sections forming integral extensions of said center section extending at an angle thereto in a direction substantially normal to the thrustdelivered to said brush with the extended ends thereof pivotallyjoined to said .carrier and having an intermediate portion of :each :leg bearing against one of said carrier abutments on the side of said spring opposite that engaging said brush, a set of bendable terminal leads havingan end of each embedded at .the time of molding within said molded base in electrical contact with one of said concentric tracks and having shank -portions emerging and extending outwardly from said base that terminate in free ends suitable for electrical connection with circuit components and leads, .and terminal lead supporting means of insulating material .closelysurrounding the shank of each lead for a substantial distance beyond the point of emergence -.of-each to isolate the embedded ends of the leads from bending :forces applied to the free ends thereof.

4. In a variable resistance apparatus the combination comprising a base of insulating material; .a circular track of resistance material supported :by said base having an exposed contact face; ,a collector track of conducting material supported by said base spaced from said resistance track and having .an exposed contact face substantially concentric with said resistance track; a rotatable brush carrier in facing relation to the exposed contact faces of said tracks having a pair of spaced spring receiving sockets therein and having a pair of abutments projeeting toward said base ,adapted to' act spring fulcrums, each fulcrum .being adjacent one of said sockets and disposed onthesame side .ofaline extending between the sockets; a conducting brush held by and rotatable with said brush carrier having contact projections resting in sliding engagement with said resistance and collector tracks to electrically bridge the same, and having a back face disposed away from said base; and tan .elongated staple shaped spring ;interposed'betWeen-said brush and said brush carrier with a central section :bearing against the back of said brush to urge the same into contacting engagement with said tracks, .and with a pair of elongated leg sections extending from said center section on opposite sides of the axis of rotation with the ends thereof secured bysaid sockets and the intermediate portions of the spring legs in .bearing engagement :With and fulcrumed on said abutments to exert resilientpressure on said brush in the direction ofsaid base.

5. In a variable resistance apparatus the combination comprising a molded base of insulating material with a rear portion of circular cross section terminating in a rear face upon which is exposed a circular resistance track and a circular collector track spaced from and substantially concentric with said resistance track, each track being formed :of conducting particles integrally molded within said base; .a heat extractor of relatively high heat conductivity formed of thin sheet :material cylindrically curved to match the vcircularresistance track which is embedded within said base with the generatrix of the surface of the sheet being normal :to the plane of said tracks and with an after edge in close proximity to said resistance track adapted to receive heat therefrom, said heat. extractor having emerging portions to the front of said base; a front housing portion of relatively,-high heat conductivity engaged in heat conducting relation with said protruding portions of said heat extractor; an insulating shell .held between said base and said front housing rportion turning around the sides of said molding aand extending in the form of a rim spaced frormsaid tbasenearvvardly'of said rear face of said base; a brush carrier rotatably mounted for movement about an axis substantially concentric with said tracks mounted in facing relation :with the rear face of said base with a face area substantially coextensive with said rear face and received Within .the rim of said insulating shell to enclose said collector and resistance tracks within an insulating jacket comprising said .base to the front of the tracks, said rim to the peripheral outside of said tracks and said carrier :to the rear of said tracks; a brush held in bridging contact with said-tracks by said brush carrier androtatable therewith; and an outer protective housing of heat conductive material enclosing said base, brush holder and rim and secured to said front housing portion in heat conductive relation thereto.

6. In ;a variable resistance apparatus the combination of a base of insulating material with a rear face presenting an :arcuate resistance track, an arcuate metallic insert of thin sheet material embedded in said base with the surface of the material extendin substantially normal to the rear face to present an edge in close proximity to said resistance track and projecting forwardly from said edge with .a set .of attachment ears protruding from the front of the base, .an insulating shell having a front portion abutting'the :frontof the :base and turning rearwardly along .side :the :base with a .rim spaced outwardly from the :base, a metallic front cover abutting said insulating shell with said attachment .ears extending therethrough and clinched .over the front cover to rigidly secure the cover, base and shell .in assembly, a metallic shaft supporting member seated upon said front cover in heat conducting relation :thereto, a carrier assembly including a shaft borne by said member that extends rearwardly through said base and a contact brush mounted on the shaft in contacting relation with the resistance track, and a cover joined to said front cover enclosing the base and shell with a gap between the cover and the rim of the shell.

7. In a variable resistance apparatus the combination comprising a molded base of insulating material having a circular collector .track and a circular resistance track spaced from and substantially concentric with said collector track :each formed of conducting particles molded integrally with .said base, a brush carrier rotatably mounted for movement about an axis substantially concentric with said .tracks having a pair of abutments adapted to be spring 'fulcrums, :a brush held by and rotatable with said brush .carrier in bridging contact with said tracks, a spring interposed between said brush and said carrier having a straight center section bearing against the side of said brush opposite said tracks and a pair of leg sections forming integral extensions of said center section extending at an angle thereto in a direction substantially normal to the thrust delivered to said brush with the extended ends thereof pivotally joined to said .carrier and having an intermediate portion of each leg bearing .against one of said carrier abutments on the side of said spring opposite that engaging said brush, and a set of terminal leads having an end of each embedded within said molded base in electrical contact with one of said concentric tracks and extending outwardly fromsaid .base to terminate-in free ends suitable for electrical connection with circuit components and leads.

8. In a variable resistance apparatus the combination comprising .a base of molded insulating material having an arcuate resistance track on one face thereof formed with oonductivepart'icles integrally molded within the base to form .a :unitary whole; an arcuate metallic heat 2,798,189 9 1o extracting insert comprising a thin sheet embedded within ductive member secured to the heat extracting insert by said base at the time of molding that is cylindrically clinching the attachment ears over the member. formed with the generatrix of the cylindrical configuration being normal to the plane of the resistance track References Cited in the file of this Pawnt and the generatrix following an arcuate path substantially 5 UNITED STATES PATENTS like that of the resistance track, said metallic insert having an edge that is within the base in close proximity to 52 et z 52 the resistance track from which edge it extends to the P side of the base opposite the track to emerge therefrom 2514682 Tenkamp July 1950 with a set of attachment ears; and a metallic heat con- 10 

